1. Field of the Invention
The subject invention pertains to the field of short pulse generators and, more particularly, to generators for providing high amplitude high repetition rate pulses with fast on-off times, and pulse duration in the order of nanoseconds.
2. Description of the Prior Art
Early prior art short pulse generators utilized a mercury relay switch to discharge a transmission line which had been charged to a high voltage through a long time constant. This technique is capable of providing subnanosecond pulses with rise times in the order of 100 picoseconds at peak pulse voltages in the order of several hundred volts. These switches, however, cannot be operated at very high duty cycles because of mechanical limitations of a vibrating reed used therein for the switching operation. Additionally, the mechanical contacts tend to deteriorate, resulting in jittery and noisy pulses. Since the deterioration of the mechanical contacts is proportional to the number of times the contacts are being opened and closed, the life of the switch is inversely proportional to the duty cycle.
Another technique employed in the early prior art utilized Hertzian (spark gap) generators. These devices can supply pulses with amplitudes in the order of 1,000 volts and rise times in the order of 100 picoseconds at repetition frequencies above 200 Hertz. Hertzian generators however, have a lifetime limited by the width of the spark gap which determines the width of the generated pulse.
One obvious solution to the mechanical and lifetime limitations of the mercury and Hertzian generators is to utilize solid state devices which are rapidly switchable and extremely reliable. It is well known that tunnel diodes, avalanche transistors, and step recovery diodes can be used to generate a series of impulse functions at a repetition rate substantially equal to the frequency of a driving function. Nanosecond pulses with rise times in the order of 25 picoseconds are achievable with tunnel diodes. These generators, however, are low amplitude devices achieving levels only in the order of 0.5 volts. Short pulses with amplitudes as high as 1,000 volts have been generated with a series stack of avalanche transistors. The rise and fall times of these generators is in the order of 6 nanoseconds, which for many applications is too slow. A significant improvement in the pulse rise time has been achieved with the combination of avalanche transistors and step recovery diodes. These devices exhibit rise times in the order of 100 picoseconds, but provide peak amplitudes only in the order of 25 volts. A device, comprising a multiplicity of serially coupled step recovery diodes, capable of providing pulse widths of 200 picoseconds and pulse amplitudes in excess of 180 volts is disclosed in U.S. Pat. No. 3,832,568 issued Aug. 27, 1974 to C. C. Wang and assigned to the assignee of the present invention. The high pulse amplitudes of this device, however, are achieved only when the pulse generator is operating into a high impedance resonant load. Additionally, pulse repetition rate of the device is limited to frequencies in the order of 40 kHz.